Such trusses are for instance used to a large extent both for fixed and floating platforms for drilling for, and production of, oil on the sea floor. For some types of platforms the entire supporting structure consists of such trusses, and for other types the truss may form part of the platform deck and/or a transition between the platform deck and its supporting structure.
As mentioned above, the members of such trusses consist mainly of tubes which usually have considerable diameters, often of an order of magnitude of 1 m. In addition, tubular supporting columns of even greater diameter are often used.
The members of the truss, often consisting of steel tube, are attached to joints at their ends, and this occurs according to conventional methods by welding the ends of the tubes together and/or to tubes running continuously through the joint.
These conventional methods for connecting the tubes in joints give rise to very considerable problems both with regard to the performance of the fabrication work and with respect to the theoretical calculations of the strength and probable life of the truss. A further element of uncertainty of considerable magnitude is due to the anisotropic properties of the tubular material.
Since most of the tubes which meet in a joint run at an angle with respect to each other, complicated adjustment work must be performed before the tubes can be welded together. The welding is in itself very difficult and time consuming due to large material thickness, narrow angles and difficult access. In most cases, the welding work can only be performed from the outside and, due to the access problems, the control of the welds must be done as the work progresses, thus braking up the welding work. The welding work is in itself so difficult that each welder must qualify especially for such tube connections. Stress relieving locally of the welds is very difficult due to the geometry of the joints and normalizing is not possible.
When tubes are welded together in a joint, it is not possible to avoid forces and stresses occuring transversely of the longitudinal direction of the tubes. Since the tubes consist of rolled plate material, this means that the material is loaded perpendicularly to the rolling plane, and in this direction the properties of the material are poorer than in the rolling plane, i.e., in other words, the material is anisotropic. This is among other things due to the fact that relatively soft impurities, which have a tendency to collect at the grain boundaries, are rolled out to flakes having poor strength and fatique characteristics. When stress occurs transversely of the rolling plane, cracks and laminar tearing easily occur in the rolling plane. This cracking may occur during the welding, or it may occur delayed several years after the structure has been put in service and may thereby escape the quality control. Such delayed cracking is considered very dangerous for the function of the joint.
Calculation of a conventional joint with respect to material thickness, fatigue etc. is extremely difficult even using modern computer techniques, and an exact calculation of every joint in a structure is practically an impossible task. In a welded connection between tubes, an extremely complicated stress distribution will occur due to the difference in tube stiffness in the axial and radial directions, and there may occur very large stress concentrations which can not be predicted with any degree of certainty. Reinforcing plates, transitions, acute angles etc. give rise to unpredictable stress concentration factors which further contribute to the uncertainty of the theoretical result.
The result is that simplified methods must be used, and the simplest of these is based on empirical calculation methods assuming that forces from subordinate members are taken up as membrane stresses in the main members. For this type of joint it is not practically possible to determine more than the minimum rupture load without regard to fatigue due to variations in diameters, thicknesses, angles, materials etc.
For larger joints it will be necessary to use internal and external reinforcing plates which transmit the member forces without the aid of membrane forces. The calculations must also here be based on the minimum rupture load for static loading where advantage is taken of the yield properties of the steel. Calculation of local stress peaks is not practically possible, and instead, local structural details are fabricated in a manner shown by practical experience to be advantageous for the purpose of avoiding fatigue, i.e., rounded corners, ground welds, no sharp edges etc.
Conventional calculation of trusses assumes that the members are movably supported in hypothetical joints. Tubular cross sections have relatively high moments of inertia, however, and new calculations by means of computer show considerable additional stresses due to the mutual fixing of the members. These additional stresses increase for the known tube joints because reinforcements make the joints stiffer and the degree of fixing of the members higher. It should be evident that calculation of such additional stresses for all joints in a truss would be quite a formidable task.
Calculation of stresses necessitates knowledge of both the existing forces and the surfaces over which these are distributed. Some tube joints are so complicated that it is necessary to use special computer programs in order to calculate the cross-sectional areas. Model tests will to a certain extent be able to contribute to reducing the uncertainty surrounding the stresses calculated, but here one will encounter the difficulty that making models to a scale sufficiently small to be suitable for the loads that can be obtained in a laboratory becomes very complicated.
The conventional tube joint thus becomes a very expensive construction because it must be over-dimensioned due to the uncertain stress calculations, because expensive materials having reduced tendency of laminar tearing must be used, and because time consuming and difficult welding procedures must be employed. The purpose of the invention is to provide a joint for trusses of the type mentioned in the introduction, for which the above-mentioned drawbacks and deficiencies are eliminated or greatly reduced.